1. Field of the Invention
The present invention generally relates to a transmission controller, a wireless base station and a method of controlling a transmission rate, and particularly relates to a transmission controller, a wireless base station and a method of controlling a transmission rate, in which a downlink transmission rate of a mobile station is controlled based on downlink transmission quality.
2. Description of the Related Art
In the WCDMA (Wideband Code Division Multiple Access) method being adopted in third generation mobile communications systems, a communications channel engaging in communications receives interference (Multiple Access Interference: MAI) from other communications channels and interference from multi-paths in the communications channel itself (Multi-Path Interference). A mobile communications system to which this WCDMA method is applied is a system in which such interference as described above limits the subscriber capacity of the system. Therefore, transmitting with the least power possible while allowing each individual channel to achieve predetermined quality enables an increase of the wireless circuit capacity.
In the current WCDMA method, a transmission power control method which enables an increase of the wireless circuit capacity by maintaining the transmission power to a necessary minimum is used, the transmission power control method further accounting for conserving power.
The transmission power control method used in the WCDMA method may be categorized into open-loop transmission power control (open-loop power control) and closed-loop transmission power control (closed-loop power control).
(Open-Loop Transmission Power Control)
In a mobile communications system, an uplink common-control channel is not a channel used in a pair of uplink and downlink circuits, so that applying closed-loop control is not possible. Therefore, open-loop transmission power control is used. In a mobile station, the downlink propagation loss using a downlink common-control channel is estimated so that uplink transmission power is determined based on that estimated value. Also for each individual channel to which closed-loop transmission power control is applied, an initial transmission power setting is usually determined using the open-loop.
(Closed-Loop Transmission Power Control)
FIG. 6A and FIG. 6B illustrate the concept of closed-loop transmission power control of the uplink and downlink circuits. FIG. 6A illustrates the concept of uplink closed-loop transmission power control, while FIG. 6B illustrates the concept of downlink closed-loop transmission power control. In closed-loop transmission power control, for both the uplink and downlink circuits, a measurement at the receiving side (a wireless base station 30, 40, or a mobile station 15, 16) of the quality of the communications channel is performed and a transmission power control (TPC) bit is transmitted using a return channel (or a control channel of layer 1 transmitted with a communications channel) so that a communications channel receiving transmission fulfills a desired quality. In other words, when a quality value of the communications channel measured as described above is greater than a target value, a transmission-power control bit (UP) which raises the transmission power is transmitted, whereas, when the quality value measured is less than the target value, a transmission-power control bit (Down) which lowers the transmission power is transmitted. Then, the mobile station and the wireless base station which receive the transmission-power control bit as described above change the transmission power based on that received transmission power control bit.
FIG. 7 is a receiving-block diagram of dual closed-loop control applied to a wireless base station and a mobile station using the WCDMA method. This dual closed-loop control consists of 2-stage loops of 1) inner-loop control and 2) outer-loop control. Below, operations of 1) the inner-loop control and 2) the outer-loop control are described.
(Inner-Loop Control)
In the inner-loop transmission power control of an uplink (or a downlink) communications channel, in a wireless base station (or a mobile station) a received baseband signal is despread at a despreading section 51 and Rake-received at a Rake receiver 52 so as to have the received SIR (Signal-to-Interference Ratio) measured at a SIR-measuring section 53. Then, the received SIR measured and a target SIR from a target SIR section 55 are compared at a comparative-decision section 54 so that, at a TPC-bit generating section 56, for a received SIR measured below the target SIR an “UP” command, and for the received SIR measured equal to or above the target SIR a “DOWN” command, is generated as a TPC bit so as to be mapped to a transmitting-side control channel and transmitted. At a mobile station (or a wireless base station), the TPC bit is received so as to cause a change of the transmission power by 1 dB based on the decoded result. Such closed-loop transmission power control is performed every slot period (0.667 ms).
(Outer-Loop Transmission Power Control)
In the inner-loop control as described above, while control so as to cause the received SIR of the communications channel to be a certain target value is performed, in the outer-loop control, a target SIR is controlled so as to cause the communications quality (for example, BER: Bit Error Rate, BLER: Block Error Rate, or FER: Frame Error Rate) to be a certain target value. More specifically, at a long-term quality measuring section 57, the communications quality as described above is measured for a certain long term (from a few 100 milliseconds to a few seconds) so that the measured value and a target quality value from a target quality section 58 are compared at a comparative decision section 59. At a target SIR section 55, based on the comparative decision result of a comparative decision section 59, a target SIR is set so as to cause the communications quality as described above to be the target quality.
Thus, in the related art, the transmission power is controlled in accordance with the dual closed-loop control method.
Furthermore, there exists a method of appropriately controlling a transmission rate depending on the circuit state so as to enable appropriate control of the transmission power of a wireless base station relative to a mobile station (for example, refer to Patent Document 1).
Patent Document 1
JP2003-023395A
Non-Patent Document 1
3rd Generation Partnership Project Technical Specification Group Radio Access Network, 23.214 Physical layer procedure (FDD), September, 2002.
As described above, in the mobile communications system based on the CDMA method, the transmission power control which maintains the receiving quality to a predetermined quality is mandatory. On the other hand, changing transmission channel characteristics causing a degradation in the transmission quality (or a degradation of the propagation environment) causes the transmission power value necessary to fulfill a predetermined receiving quality to be increased to a predetermined value (or the maximum transmission power) so that a further raising of the transmission power is not possible when the predetermined value is reached. Therefore, while in the related art, control is performed in whereby the downlink transmission rate is lowered so that the transmission power capacity of a wireless base station is not exceeded, in such a case, there is a problem in that the processing burden at the time of the transmission rate control becomes large when simultaneously controlling all mobile stations requiring control.
Furthermore, in the method of controlling the transmission rate as described in Patent Document 1, the fact that a control process to lower the transmission rate at the time of a poor circuit state is performed, thereby causing appropriate control of the transmission power of a wireless base station relative to a mobile station, is disclosed. However, this related-art method does not reduce the processing burden at the time of controlling the transmission rate.